APPLICATION OF CADMIUM TELLURIDE DETECTORS TO HIGH-ENERGY COMPUTED-TOMOGRAPHY

15 years ago, cadmium telluride detectors have been investigated in our laboratory as possible detectors for medical scanners [1]. Today most of these machines are using high pressure xenon gas as multicells detectors, and BGO or CdWO4 scintillators for industrial computerized tomography. Xenon gas detectors are well suited for the detection of 100 keV X-rays and enables one to built 1000 cells of homogeneous detector with a dynamic range of 3 decades. BGO and CdWO4 scintillators, associated with photomultipliers or photodiodes, are used for higher energy (400 keV). They present a low afterglow and a dynamic range of 4-5 decades. Nondestructive testing of very absorbing objects (e.g. 2 m diameter solid rocket motor) by X-ray tomography requires much higher energy X-rays (16 MeV) and doses up to 12000 rad/min at 1 m. For this application cadmium telluride detectors operating as photoconductors are well suited. A prototype tomograph machine, able to scan empty set 0.5 m high density objects has been realized with 25 CdTe detectors (25 x 15 x 0.8 mm3). It produces good quality 1024 x 1024 tomographic images.